1. Field of Invention
The invention relates to the field of metal and non metal forming, and particularly to an improvement to the method of making sandwich structures by a combination of selective joining and, accordion expansion.
2. Description of Prior Art
It has been known for many years that certain metals, such as titanium, and other alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency toward necking. This capability is exhibited by only a limited number of metals and alloys, and within limited temperature and strain rate ranges. For example, some titanium alloys, such as Ti-6Al-4V, have been observed to exhibit superplastic characteristics.
Until the advent of viable superplastic forming techniques, taking advantage of this property to form complex configurations requiring large tensile elongations was extremely difficult or, in some instances, not possible. Significant breakthroughs in superplastic forming were made by Hamilton, et al., which are disclosed in U.S. Pat. Nos. 3,934,441, Controlled Environment Superplastic Forming, and 4,181,000 Method For Superplastic Forming incorporated into this specification herewith by reference. Simplified, the process involves placing a metal blank workpiece over a cavity in a chamber. The workpiece is heated to a temperature where it exhibits superplastic characteristics after which differential pressure is applied to the workpiece, causing it to stretch and form into the cavity.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a sufficient time so as to cause co-mingling of the atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid state at or above one-half the base metal melting point. Actual times, temperatures and pressures will vary from metal to metal.
The combining of superplastic forming and diffusion bonding (SPF/DB) in the making of metallic sandwich structures has been successfully accomplished and is disclosed in U.S. Pat. No. 3,927,817, method of making metallic Sandwich Structures, by Hamilton, et al., also incorporated by reference.
Basically, the Hamilton, et al. method for making metallic sandwich structures involves fabricating the structures from a plurality of metal blank workpieces. One or more of the blanks are coated with stopoff in selected areas not to be diffusion bonded. The blanks are positioned in a stacked relationship and placed in a die assembly wherein the stack is clamped at its periphery forming a seal thereabout. The stack is diffusion bonded together in the uncoated areas by the application of pressure, and at least one of the blanks is superplastically formed against one or more of the die surfaces forming the sandwich structure. The core configuration is determined by the location, size and shape of the joined areas.
One of the problems with the Hamilton, et al. process is that there is no disclosure of a method of forming separate vertical webs or reinforcements. Such webs can have structural advantages. In some applications (regardless of strength requirements) vertical webs are a necessity, for example, in air ducts where the vertical webs act as guides for the air flowing therethrough. Another problem with the Hamilton, et al., process is that it is limited to metallic alloys having superplastic characteristics.
A prior art example of forming vertical webs in a sandwich structure is disclosed in U.S. Pat. No. 3,834,000, Method of Manufacturing a Multi-Webbed Expanded Steel Panel, by G. D. Miller. Miller's method involves forming a stack composed of a plurality of web bars between cover plates. The top and bottom surfaces of the bars are coated with a stopoff, excepting opposite edges, i.e., one end of the top surface and the opposite edge of the bottom surface. The assembly is roll diffusion bonded and expanded by pulling the cover plates apart (for example, by use of vacuum cups). The problem with this process is that in order to expand the stack the top cover must be pulled both upward and to the side. Thus, the stack could not be expanded within dies by means of internal pressure. This is particularly true if the stack is to be clamped at its periphery within the pair of dies.
Other methods of making sandwich structures with vertical webs typically require that the webs and cover sheets be fabricated separately. Joining of the two is, typically, accomplished by diffusion bonding, brazing, welding or mechanical fasteners. It is readily apparent that such methods are time consuming and, therefore, expensive.
Thus, it is a primary object of this invention to provide a method of making sandwich structures by selective joining and accordion expansion.
It is a further object of this invention to provide a method of making sandwich structures by a combination of diffusion bonding, accordion expansion, and superplastic forming.
It is a still further object of this invention to provide a method of making sandwich structures having vertical internal webs.